This invention relates to a system for supplying supplemental air into the exhaust system of an internal combustion engine, and more particularly to an improvement in the same system of the type wherein the supplemental air is inducted into the exhaust system by the action of exhaust pressure pulsation generated in the exhaust system.
In connection with internal combustion engines provided with exhaust systems of the type wherein unburnt constituents in exhaust gases are burnt to discharge clean exhaust gases into the atmosphere, it is well known that supplemental air is supplied to the exhaust system to support burning of the unburnt constituents in the exhaust gases. This supplemental air is, in general, called "secondary air" and accordingly is referred to as "secondary air" hereinafter. It is also known in the art, that the atmospheric air as the secondary air is intermittently induced into the exhaust system by using a reed-type check valve or reed valve which is operated by the action of exhaust pressure pulsation generated in the exhaust system.
However, a secondary air supply system using such a reed valve has been encountered the difficulties in which exhaust noise is transmitted through a secondary air supply passage back toward the reed valve and then the exhaust noise is radiated into a passenger compartment to produce uncomfortable sound.
The experiments by the inventors revealed that the exhaust noise having the frequency ranging from 60 to 120 Hz is predominant in a four-cylinder engine, and the exhaust noise having the frequency ranging from 100 to 175 Hz is predominant in a six-cylinder engine. It will be understood that the main parts of such exhaust noises are within a relatively low frequency range.
In this regard, it has now been in practice to mount a reed valve on the body of an engine air filter and form a tail pipe of the check valve along the inner wall surface of the body of the air filter, in order to decrease the exhaust noise.
However, such a tail pipe serving as an air induction passage is unavoidably limited in length and the cross-sectional area in due consideration of the inner space of the air filter and the necessary amount of secondary air. Accordingly, the exhaust noise within a low frequency range cannot sufficiently dissipated althrough the exhaust noises within medium and high frequency ranges can be dissipated to a considerable extent.
Otherwise, in order to decrease the exhaust noise, it has also been proposed to increase the cross-sectional area of a part of a secondary air induction passage to form a so-called expansion chamber for dissipation of the exhaust noise. In this case, a considerably long expansion chamber is required to dissipate low frequency noise. However, it is difficult to form such a long expansion chamber in view of the space in the engine room.
In this regard, it has also been proposed to decrease the cross-sectional area of a part of the air induction passage to increase the noise siliencing effect. However, with such a proposition, the flow resistance of the inducted air is increased to decrease the amount of secondary air inducted in the exhaust system, unavoidably lowering the cleaning effect to the unburnt constituents in the exhaust gases.